Certain subsurface formations may include organic matter, such as shale oil, bitumen, and/or kerogen, which has material and chemical properties that may complicate production of fluid hydrocarbons from the subsurface formation. For example, the organic matter may not flow at a rate sufficient for production. Moreover, the organic matter may not include sufficient quantities of desired chemical compositions (typically smaller hydrocarbons). Hence, recovery of useful hydrocarbons from such subsurface formations may be uneconomical or impractical.
Heating of organic matter-containing subsurface formations may be particularly useful in recovering hydrocarbons from oil shale formations. For example, heating organic matter-containing subsurface formations pyrolyzes kerogen into mobile liquids and gases, and may reduce the viscosity of heavy oil to enhance hydrocarbon mobility.
One method to heat a subsurface formation is to conduct electricity through the formation and, thus, resistively heat the subsurface formation. This method of heating a subsurface formation may be referred to as “bulk heating” of the subsurface formation. Bulk heating of the subsurface formation may be accomplished by providing electrode assemblies in the subsurface formation and conducting electricity between pairs of the electrode assemblies. The electrode assemblies may be contained in wellbores and/or manmade fractures, and the electrode assemblies may include electrical conductors, such as metal rods and granular electrically conductive materials.
As heating occurs in subsurface regions between the pairs of electrode assemblies, the electrical conductivity (or alternatively, resistivity) of the subsurface regions may change. This change in the electrical conductivity (or resistivity) of the subsurface regions may be due to physical and/or chemical changes within the subsurface regions, for example, due to temperature sensitivity of the electrical resistance of the native rock, due to native brine boiling off, and/or due to pyrolysis (and/or coking) of native hydrocarbons.
Heating a subsurface region via electrical conduction through the subsurface region may not occur uniformly and may suffer from instabilities, in particular if conductivity within the subsurface region increases strongly with increasing temperature. The conductivity increase within the subsurface region may result from pyrolysis occurring and may lead to the formation of electrically-conductive coke or other graphitic materials. When electrical conductivity increases strongly with increasing temperature, hotter regions will become even hotter, since electricity may channel through the hotter (and more conductive) regions. Ultimately, this positive correlation between temperature and electrical conductivity may lead to the formation of a narrow, highly-conductive shunt between the electrode assemblies that will short-circuit the electrical flow between the electrode assemblies. Although the electrode assemblies may be large in extent or area, the bulk of the electrical flow may occur through a very small zone and heating of the subsurface region between the electrode assemblies may be quite uneven. This phenomenon is analogous to viscous fingering that may occur when a low viscosity fluid is driven through a higher viscosity fluid. In bulk heating, the tendency for shunting instabilities to occur and the rate of shunt growth may be dependent on the heating rate and the extent to which electrical and physical property heterogeneities exist within the subsurface regions.
Conventional methods to minimize the effects of subsurface shunts during bulk heating include disconnecting at least one of the affected electrode assemblies (electrode assemblies that conduct current into a shunted region). Where an affected electrode assembly serves more than one subsurface region, disconnecting the affected electrode assembly stops the generation of heat in the shunted region and all other (unaffected) subsurface regions served by the affected electrode.
In view of the aforementioned disadvantages, there is a need for alternative methods for mitigating the effects of subsurface shunts during bulk heating of a subsurface formation.